1. Field of the Invention
This invention relates to an electronically controlled power steering apparatus which controls the steering assist amount of a steering mechanism of a vehicle, and more particularly to an electronically controlled power steering apparatus of the type wherein the aimed assist amount is set in response to lateral acceleration of the vehicle in accordance with a fuzzy rule.
2. Description of the Related Art
In recent years, power steering apparatuses have varied widely for assisting the force (hereinafter referred to as steering wheel operating force or steering force) for operating a steering wheel. Particularly, hydraulic power steering apparatuses which make use of a hydraulic cylinder mechanism to hydraulically assist the steering wheel operating force are popularly employed as such power steering apparatus. Also electrically operated power steering apparatuses wherein the steering wheel operating force is assisted by an electric motor have been developed.
Such power steering apparatuses as described above allow steering of a vehicle, for which high steering wheel operating force is required, such as, for example, a large size vehicle or a vehicle which employs wide tires on wheels for steering, to be performed with a low steering wheel operating force, eliminating so-called heavy operation of the steering wheel.
When the vehicle speed is low such as upon garaging, generally the steering operation is desired to be performed with a lower steering force. On the other hand, When the vehicle is running at a high speed, if the steering operation is very light, then running of the vehicle becomes unstable, and accordingly, the steering operation is desired to be rather heavy. Thus, a vehicle speed responsive power steering apparatus has been developed wherein the steering wheel operation is controlled in response to the speed of the vehicle such that, when the vehicle runs at a low speed, the steering assist amount is set to a comparatively high value so as to make the steering wheel operation lighter, but when the vehicle runs at a medium or high speed, the steering assist amount is set to a comparatively low value to make the steering wheel operation heavier.
In one such vehicle speed responsive power steering apparatus, a vehicle speed sensor is provided on the vehicle while a valve for adjusting hydraulic oil to be supplied to a power cylinder is provided in a hydraulic system of the hydraulic power steering apparatus. Operation of the valve is controlled in response to a vehicle speed detected by the vehicle speed sensor to adjust the steering assist amount. The vehicle speed responsive power steering apparatus of the type just mentioned is called an electronically controlled power steering apparatus.
In the following, construction of an exemplary conventional electronically controlled power steering apparatus will be described with reference to FIGS. 14 to 16.
Referring to FIGS. 14 to 16, an input shaft 11 is fitted for rotation in a casing 25 by means of bearings and is connected to receive steering force from a steering wheel (not shown). A pinion 12 is mounted for relative rotation at a lower end of the input shaft 11 with a bush or a like element (not shown) interposed therebetween.
A torsion bar 15 is located in the hollow inside of the input shaft 11. The torsion bar 15 is coupled at an upper end thereof for integral rotation to the input shaft 11 by way of a pin or a like element while it is not restrained at a lower end thereof by the input shaft 11.
The pinion 12 is held in serration coupling engagement with the lower end of the torsion bar 15 so that the steering force inputted to the input shaft 11 may be transmitted to the pinion 12 by way of the torsion bar 15. The pinion 12 is held in meshing engagement with a rack 13 so that the steering force may be transmitted to the rack 13 by way of the pinion 12 to move the rack 13 in its axial direction (in a direction perpendicular to the plane of FIG. 14) to steer wheels of the vehicle (not shown).
A power steering hydraulic cylinder 14 includes a cylinder 14A mounted on a member on the body of the vehicle, and a piston 14B provided intermediately of the rack 13 for movement in an axial direction in the cylinder section 14A together with the rack 13. The inside of the cylinder 14A is partitioned leftwardly and rightwardly by the piston 14B into a pair of oil chambers 14C and 14D.
A rotary valve 16 is provided for driving the hydraulic cylinder 14. Operating oil is supplied into or discharged from the left or right oil chamber 14C or 14D of the hydraulic cylinder 14 by opening or closing motion of the rotary valve 16 to provide steering assist force to the rack 13.
The rotary valve 16 is interposed between the input shaft 11 and the pinion 12 and is opened or closed in response to a difference in phase between the input shaft 11 and the pinion 12. In particular, when steering force is inputted to the input shaft 1I, the input shaft 11 is rigid and presents little distortion, but the torsion bar 15 transmits the steering force to the pinion 12 while presenting some distortion. Consequently, the pinion 12 presents a difference in phase (angular position) with respect to the input shaft 11. The rotary valve 16 is opened or closed so as to produce required steering assist force in the steering direction in response to the difference in phase.
A plurality of reactive force plungers 17 for providing, upon steering, steering reactive force to increase the steering force (that is, steering reaction) are provided on an outer periphery of a lower portion of the input shaft 11 such that they surround the outer periphery of the input shaft as seen in FIG. 16. The reactive force plungers 17 receive, at chambers 17A at back portions thereof, hydraulic oil supplied thereto under the control of a hydraulic pressure control valve 18 to restrain the input shaft 11 to provide steering reactive force in response to the hydraulic pressure. The chambers 17A are communicated with an oil reservoir 24 by way of respective return orifices 22.
The hydraulic pressure control valve 18 is provided next to and extends in parallel to the input shaft 11 in the casing 25 as shown in FIG. 15 and includes a plunger 18A fitted for upward and downward sliding movement in the casing 25, a solenoid 19 for exerting upward axial force to the plunger 18A, and a spring 20 for normally biasing the plunger 18A downwardly.
The plunger 18A has a pair of oil passages 18B and 18C communicated with the oil reservoir 24, an annular oil passage 18D for communicating with the oil pump 23, another annular oil passage 18E for communicating with the chambers 17A of the reactive force plungers 17, and an oil passage 18F for communicating the annular oil passages 18D and 18E with each other. In short, operating oil under high pressure from the oil pump 23 is supplied from the annular oil passage 18D into the chambers 17A of the reactive force plungers 17 by way of the oil passage 18F and the annular oil passage 18E.
Upon steering, for example, while the vehicle stops or is running at a low speed, maximum current is supplied to the solenoid 19. Consequently, the plunger 18A is moved upwardly to its highest position in which the annular oil passage 18D is not communicated with the oil pump 23 and supply of oil to the chambers 17A of the reactive force plungers 17 is stopped. Consequently, the reactive plungers 17 do not restrain the input shaft 11, and steering can be performed with light-force.
On the other hand, for example, while the vehicle is running at a medium or high speed, the current supply to the solenoid 19 is decreased in response to an increase of the vehicle speed. Consequently, when the steering wheel is at its neutral position, the axial force of solenoid 19 on the plunger 18A decreases as the current decreases, and as the axial force decreases, the plunger 18A is moved down so that the annular oil passage 18D is communicated with the oil pump 23 to allow oil to be supplied to the chambers 17A of the reactive force plungers 17.
In this condition, the reactive force plungers 17 restrain the input shaft 11 to hold the steering Wheel at its neutral position. Then, if the steering wheel is moved a little from the neutral position, then the output of the oil pump 23 tries to rise. In this instance, the discharging pressure of the oil pump 23 acts upon the chambers 17A of the reactive force plungers 17 almost without being controlled by the hydraulic pressure control valve 18. Accordingly, in the proximity of the neutral position of the steering wheel, the steering force is increased and a sufficient response of the steering wheel at the neutral position is obtained, resulting in a feeling of stability of the steering wheel in the neutral position.
Upon steering while the vehicle is running at a medium or high speed, the output of the oil pump 23 rises, within an ordinary steering range, to increase the steering assist amount in response to steering of the steering wheel, that is, in response to an increase of the steering force. Meanwhile, the discharging pressure of the oil pump 23 acts upon the chambers 17A of the reactive force plungers 17 while being controlled by the hydraulic pressure control valve 18. Accordingly, the reactive force plungers 17 act to restrain the input shaft 11 to increase the steering response (steering force).
As a result, upon steering when the vehicle rune at a medium or high speed, the steering force is increased by an amount corresponding to the action of the reactive force plungers 17 as compared with the steering force acting upon steering when the vehicle stops or is running at a low speed. In short, the steering response is increased and a stable steering feeling is obtained. Particularly, when the current supply to the solenoid 19 is decreased in response to an increase of the vehicle speed, the steering assist amount decreases and the steering force (steering response) increases, and consequently, a more stable steering feeling can be obtained.
The steering assist characteristic can be controlled by adjusting the current to be supplied to the solenoid 19 in this manner. As seen from FIG. 16, the current to be supplied to the solenoid 19 is set by a control unit (control means) 30 in response to, in addition to vehicle speed information from a vehicle speed sensor 31, for example, mode setting information from an EPS (electronically controlled power steering) mode changeover switch 32 and an engine rotation signal from an engine speed sensor 33 or a like element to control the solenoid 19.
In particular, the EPS mode changeover switch 32 can selectively set a normal mode and a sport mode in which the steering force is increased beginning at a lower speed than in the normal mode. When one of the modes is set, the control unit 30 controls the steering assist characteristic of the power steering mechanism in accordance with the set mode. For example, when the sport mode is set the current to be supplied to the solenoid 19 is adjusted in response to vehicle speed information so as to present such an assist characteristic that the assist amount decreases gradually as the vehicle speed increases from a medium speed range of the speed V.sub.1 as seen from FIG. 17. On the other hand, when the normal mode is set, the current to be supplied to the solenoid 19 is adjusted in response to vehicle speed information so as to present such an assist characteristic that the assist amount decreases gradually as the speed vehicle increases from a little higher speed range of the speed V.sub.2 (&gt;V.sub.1).
Further, when trouble with a detection circuit is detected from vehicle speed information, an engine rotation signal or the like, the solenoid 19 is switched off to effect fail-safe control.
Another consideration is that the required steering force characteristic actually varies depending upon a running condition of the vehicle, that is, whether the vehicle is running straight forwardly or along a curve or whether the vehicle is being accelerated or braked. However, conventional electronically controlled power steering apparatus have not been successful in always providing an optimum steering feeling since they control the steering force merely in response to the speed of the vehicle as described above.
For example, when the vehicle advances to a corner, the steering force characteristic is desired to present somewhat heavy steering force so that the driver can grasp the running condition of the vehicle at that time from an appropriate variation of the steering force.
In particular, when the vehicle advances to a corner while being accelerated, the steering force characteristic is desired to present somewhat heavy steering force so that the driver can grasp the running condition of the vehicle appropriately at that time. However, with the conventional electronically controlled power steering apparatus, although the steering force characteristic presents a little heavy driving force as a result of acceleration, a sufficiently heavy driving force cannot be provided in most cases.
On the other hand, when the vehicle advances to a corner while being decelerated, the steering force characteristic is desired to present rather heavy steering force so that the driver can grasp the running condition of the vehicle then appropriately. However, with the conventional electronically controlled power steering apparatus, the steering force characteristic becomes higher by an amount corresponding to the deceleration.
Further, when the vehicle advances to a corner at a fixed speed, the steering force characteristic is desired to present a little heavier ("a little heavier" here represents heavier than "rather heavy") steering force so that the driver can grasp the running condition of the vehicles more appropriately. With the conventional electronically controlled power steering apparatus, however, the steering force characteristic does not present any variation then.
In addition to the electronically controlled power steering apparatus described above, several power steering apparatus have been proposed including a power steering apparatus disclosed in Japanese Utility Model Publication Application No. Heisei 2-41104 wherein the steering assist amount is varied when the output of one of a vehicle speed sensor and an acceleration sensor exceeds a predetermined value. Another power steering apparatus is disclosed in Japanese Patent Laid-Open Application No. Heisei 4-230472 wherein the steering assist amount is varied when it is detected from a lateral acceleration signal and a road friction coefficient that the turning condition of the vehicle approaches a critical condition. A further power steering apparatus is disclosed in Japanese Patent Laid-Open Application No. 2-182576 wherein the steering assist amount is varied in accordance with a fuzzy rule from the vehicle speed and an unevenness of the road.
However, also those power steering apparatuses fail to control the steering assist amount for the steering wheel so as to provide an optimum steering characteristic in response to a running condition of the vehicle and particularly do not always provide an optimum steering feeling, similarly to the conventional electronically controlled power steering apparatus described above.